1. Field of the Invention
The present invention relates to a counter device for a sheet photo film pack. More particularly, the present invention relates to a counter device for a sheet photo film pack capable of properly indicating a counted number of exposures taken in combination with a camera and a pack holder.
2. Description Related to the Prior Art
A sheet photo film pack has a pack housing, which contains a stack of plural sheet photo film units. A pack holder is loaded with the sheet photo film pack. The pack holder is used in combination with a large-format camera of a frame size of 4.times.5 inches (102.times.127 mm). The pack housing has a slidable lid. The use of the pack holder makes it easy to take photographs in the large-format camera one after another, because the slidable lid is only slid back and forth before each of the sheet photo film units can be readily positioned at an exposure aperture in place of a preceding exposed one of the sheet photo film units. Each of the sheet photo film units is constituted of a sheet photo film having a size of 4.times.5 inches, and a plastic sheath for containing the sheet photo film for the purpose of protection.
U.S. Pat. No. 5,710,952 (corresponding to JP-A 8-110568) discloses a sheet photo film pack, in which a counter chamber is formed in the pack housing in a box shape. The counter chamber contains a rotatable counter disk, which is rotated and stepped up by a counter advance spring plate or lever of the pack holder. One face of the counter disk, where a train of counter numerals are arranged, is provided with stepping claws of a gear shape. The counter advance spring plate of the pack holder is engaged with the stepping claws, and rotates and steps the counter disk in one direction in cooperation with the stepping claws.
The counter advance spring plate or lever is produced by cutting an end of a metal thin arm obliquely, and is bent vertically. The bent end of the arm is provided with a shape of a right-angled triangle. Each time that one exposure is taken, the counter advance spring plate is rotated from its initial position, causes the counter disk to make stepwise rotation by one step, and is rotated back to its initial position to be stand by for next stepping operation. In the returning operation, the counter advance spring plate forcibly comes back to the tip of one of the stepping claws next to the present one of them, and moves past it.
Also the counter disk has plural ratchet claws for anti-reversal operation. The ratchet claws are located on a disk face reverse to that having the counter numerals. The ratchet claws are combined with a push plate to constitute a ratchet mechanism. There is a spring portion formed integrally with the push plate, which is vertically pressed against the counter disk by the bias force of the spring portion.
However failure related to the counter advance spring plate or lever is likely to cause erroneous operation of the counter disk. Examples of the failure include burrs created with the claw drive end with low precision at the time of cutting out the counter advance spring plate, failure in precisely bending the counter advance spring plate, and failure in precisely positioning the counter advance spring plate. In the returning movement, it is likely that the counter advance spring plate does not move past one of the stepping claws next to one of them engaged with the counter advance spring plate. Backward push of the counter advance spring plate occurs to the one stepping claw. For example, the counter disk rotates in reverse by half a step. In the next stepping operation of the counter device, the counter advance spring plate pushes again the same one of the stepping claws. Accordingly there occurs a difference between an indicated counter numeral and the number of the actually exposed sheet photo film units. When all the sheet photo film units are exposed finally, the pack is erroneously enabled to take another exposure. The first exposed one of the sheet photo film units is exposed again as a double exposure.
It is also likely that the counter disk does not make enough rotation of a predetermined angle for various causes, including insufficiency in the swinging angle of the counter advance spring plate or lever due to failure in assembly of the counter advance spring plate, and errors in molding the pack housing. At this time of the insufficient rotation, the push plate remains in contact with the tip of the ratchet claws, and does not operate for preventing the reverse rotation. This is included in the causes of the above-described half-step reversal in rotation of the counter disk.
In the above-described push plate, the bias force of the spring portion is lowered with time, for example after preservation for a long time. There occurs failure in correct stepping in the counter device. If the sheet photo film pack is dropped forcibly or shocked considerably in any manner, the indicated counter numeral is likely to change as the counter disk may come to an unwanted position.
There is an adequate clearance between a slidable tray and the inside of the pack housing for the purpose of convenience in loading the pack housing. A problem lies in that the stepping of the counter device is unstable because the clearance involves a changeable relation in the distance between the counter disk and the counter advance spring plate or lever. Should the distance be too great, the counter advance spring plate is likely not to be engaged with the counter disk. The counter advance spring plate may swing without driving the counter disk, so that the counter device may not be stepped properly. Should the distance be too small, the counter advance spring plate may come to too deep a position between the stepping claws of the counter disk. In the return operation of the counter advance spring plate, the counter advance spring plate may be caught by the stepping claws, and may rotate back the counter disk in the manner of "back-stepping" of the counter device.